Other Comprehensive Income - Schedule of Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Equity [Abstract]
Other comprehensive income (loss), before tax	$ 160.7	$ (621.3)	$ (90.4)
Other Comprehensive Income (Loss), Tax	40.1	(155.2)	(22.8)
Other Comprehensive Income (Loss), Net of Tax	120.6	(466.1)	(67.6)
Before Tax Amount
Change in net unrealized gain during period	144.8	(613.1)	(113.7)
Reclassification adjustment for net losses included in net income	23.5	24.4	(1.1)
Reclassification adjustment for securities transferred from held-to-maturity to available-for-sale	(7.2)	0.2	0.0
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), before Tax	(1.3)	(26.1)	20.2
Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net	0.6	(4.9)	3.1
OCI, Debt Securities, Available-for-Sale, Gain (Loss), before Adjustment, Tax	0.9	(6.7)	4.2
Tax Expense (Benefit)
Change in net unrealized loss during period	36.1	(152.4)	(28.7)
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), Tax	(0.3)	(6.6)	5.1
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	0.3	(1.8)	1.1
Reclassification adjustment for net losses included in net income	5.8	5.5	(0.3)
Net of Tax Amount
Change in net unrealized loss during period	108.7	(460.7)	(85.0)
Reclassification adjustment for net gains included in net income	17.7	18.9	(0.8)
OCI, Debt Securities, Available-for-Sale, Transfer to Held-to-Maturity, Adjustment from AOCI for Amortization of Gain (Loss), after Tax	(1.0)	(19.5)	15.1
Stockholders' Equity Attributable to Parent	3,227.5	3,073.8	$ 1,986.6	$ 1,959.8
Net change in unamortized (losses) gains on available-for-sale investment securities transferred into held-to-maturity	(7.2)	0.2
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Tax	(1.8)	0.1
Other Comprehensive Income (Loss), Transfers From Available-For-Sale To Held-To-Maturity Securities, Net Of Tax	$ (5.4)	$ 0.1